(a) Field of the Invention
The present invention relates to a redundancy fuse block having a small occupied area and, more particularly, to a redundancy fuse block for use in a semiconductor memory device, such as DRAM, having a redundancy function.
(b) Description of a Related Art
A semiconductor memory device such as DRAM generally has a redundancy function, wherein a row (or column) of memory cells including one or more of defective memory cells is replaced by a redundancy row (column) including redundancy cells in number corresponding to the number of the memory cells to be replaced. If it is found that a memory device has a defective memory cell in a product test, the address of the defective row is stored in a redundancy circuit by selectively cutting a plurality fuses in a fuse block by a laser beam. Each pair of fuse elements in the fuse block represents a bit of row address by the on- or off-states of the fuse elements.
In operation of the semiconductor memory device, if the redundancy decoder detects that an input row address coincides with the address of the replaced row, the input row address is replaced with the redundancy row address by switching the input row address, whereby the defective row is replaced by the redundancy row.
FIG. 1 shows a conventional semiconductor device having a fuse block including fuse elements 33, made of polycrystalline silicon (polysilicon), in number corresponding to the number of bits of row address. The fuse block is received in a rectangular window 31 having longer sides perpendicular to the extending direction of each elongate fuse element 33. In this example, eight fuse elements 33 are arranged in the direction parallel to the longer sides of the rectangular window 31 with a specified space disposed between each two of the fuse elements 33. Both ends of each fuse element 33 are connected to signal lines 34 and 36 made of aluminum at the contacts 35 and 37 disposed outside the window 31. The fuse elements 33 are to be selectively cut by the laser beam within the window 31, for detecting a defective row address in the redundancy address decoder.
During cutting the fuse element 33, the laser beam is irradiated within a circular area "A" receiving therein the bridge section 33a of the single fuse element 33, whereby the specified fuse element 33 is cut at the bridge section 33a without cutting the adjacent fuse elements 33.
In the arrangement of the fuse elements 33, the circular area "A" for assuring a sufficient space between each two of the fuse elements 33 provides safe cutting of a specified fuse element 33 without miss-cutting of the adjacent fuse elements. However, the sufficient space for the fuse elements 33 enlarges the pitch of the signal lines 34 and 36 and associated transistors (not shown), thereby enlarging the overall chip size for the semiconductor device.
Patent Publication JP-A-6-310603 describes a fuse block having a reduced occupied area, such as shown in FIG. 2, wherein two out of three fuse elements 33 have parallel offsets therein. A first pair of fuse elements 33 shown at the left side, fore example, of the window 31 have right-wise offsets, whereas second pair of fuse elements 33 disposed adjacent to the first pair of fuse elements 33, with a straight fuse element 33 sandwiched between the pairs, have left-wise offsets. By deviating the circular areas "A" of the pair of fuse elements 33 from each other in the direction of the elongate fuses 33, the longer sides of the window 31 are reduced in length with the shorter sides being somewhat increased,. The circular area "A" has a radius of 3.5 micrometers (.mu.m), for example.
In the fuse block described in the publication, however, the advantage in reduction of the area for the window 31 is relatively limited, wherein the longer sides of the window 31 may be reduced from 35 .mu.m to 29 .mu.m, for example.